1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer and a facsimile device, and more particularly relates to an image forming apparatus that performs image formation employing a two-component developer comprising a toner and a magnetic carrier.
2. Description of the Related Art
Two-component development systems in which a two-component developer (hereinafter referred to simply as “developer”) comprising a toner and a magnetic carrier is carried on a developer carrier and in which development is carried out as a result of a magnetic brush being formed from the developer by magnetic poles provided within the developer carrier and a latent image on a latent image carrier being rubbed by the magnetic brush are widely known in the prior art. Two-component development systems are being widely utilized because of the simplicity of coloring they afford. When the toner density as an expression of the ratio (for example, weight ratio) of the toner and magnetic carrier in a developer in a two-component developer system is too high, blemishes and a depot in the fine resolution of the formed image occur. On the other hand, when the toner density lowers, the density of the solid image portion drops and adhesion of the carrier to the latent image carrier occurs. Accordingly, it is essential that a toner density control involving the control of a toner supply operation based on the detection of the toner density in the developer of the development apparatus to be performed to always maintain the toner density in the developer within the appropriate range.
In addition, it is essential that the image forming performed by the image forming apparatus be performed in a way that in general always produces a constant image density. Image density is principally determined by the development capability of the development apparatus. Development capability, which refers to a capability that expresses the extent to which toner can be adhered to a latent image during development, changes in accordance with, in addition to toner density, development conditions such as development potential or the toner charge amount contributing to development. A gradient (development γ) of a relational expression that describes the toner adhered amount with respect to the development potential is widely used as an index for denoting development capability. Because the image density is determined by the development capability of the development apparatus in this way, performing the toner density control alone described above to produce a toner density that is always within the appropriate range cannot produce a constant image density. In addition, even though it is comparatively easy to ensure development conditions such as the development potential are made constant, ensuring the toner charge amount contributing to development is made constant is difficult. Accordingly, there is a drawback inherent thereto in that, even if the development conditions are made constant and, in addition, a toner density control is performed to ensure the toner density is made constant, unless the development capability can be made constant a constant image density cannot be produced.
More specifically, for example when an image of low image coverage ratio is output, because the amount of toner used to develop this image is comparatively small, a small amount of toner is supplied to maintain the prescribed toner density. Accordingly, a large amount of toner is present in the development apparatus for a comparatively long time. Because the toner present in the development apparatus for a comparatively long time is subjected to an agitating action for a long time, most of the toner contributing to development is sufficiently charged to the desired charge amount. Accordingly, this gives rise to a comparatively low development capability. In contrast, when an image of high image coverage ratio is output, a large amount of just supplied new toner that has not been sufficiently charged is present (in the development apparatus), and a large ratio of the toner contributing to development is occupied by toner that has not been sufficiently charged to the prescribed charge amount. As a result, a comparatively high development capability is created. More particularly, to meet the demand for the compacting of development apparatuses that has occurred in recent years, the trend is towards as far as possible minimizing the amount of developer that is held in the development apparatus. Accordingly, for image formation performed following the output of an image of high image coverage ratio, the ratio of toner contributing to development that has not been sufficiently charged to the desired charge amount is greater. Accordingly, a comparative increase in the development capability during the image formation that follows the output of an image of high image coverage ratio is liable to be created.
In addition, based on this configuration, it is possible for the development capability when an image of low image coverage ratio is output to be higher than that when an image of high image coverage ratio is output. For example, employing a toner to which an external additive has been adhered and employing a development apparatus in which this toner creates a high stress, as a result of the toner present for a comparatively long time in the development apparatus being subjected to an agitation action for a long period, the external additive becomes either embedded in the toner surface or separates from the toner surface. Where this happens to a lot of the toner, a worsening of the fluidity of the developer occurs, the charge capability of the toner itself drops, and the toner contributing to development cannot be sufficiently charged to the desired charge amount. Accordingly, when an image of low image coverage ratio is output, because of the increase in the ratio of toner contributing to development that is not sufficiently charged to the desired charge amount, a comparatively large development capability is created. In contrast, because of the large amount of supplied toner when an image of high image coverage ratio is output, the amount of toner present for a comparatively long time in the development apparatus is small. Accordingly, the developer has good fluidity and, in addition, most of the toner has a sufficiently high charge capability. Accordingly, because the toner contributing to development can be sufficiently charged to the desired charge amount, a comparatively low development capability is created.
As is described above, differences in development capability between when an image of low image coverage ratio is output and an image of high area ratio is output are produced because of the difference in the ratio of the toner present in the development apparatus caused by the subsequent toner supply. Accordingly, there is a drawback inherent thereto in that, even if the development conditions are made constant and, in addition, a toner density control is performed to ensure the toner density is made constant, unless the development capability can be made constant a constant image density cannot be produced.
Examples of image forming apparatuses able to suppress this drawback include the apparatuses described in Japanese Unexamined Patent Application No. S57-136667 and Japanese Unexamined Patent Application No. H2-34877. In these image forming apparatuses, which comprise toner density detection means for detecting and outputting the toner density of a two-component developer of a development apparatus, a control that involves a comparison of the output value of toner density detection means and a toner density control standard value and the control of toner supply device based on the comparative result thereof so that the toner density of the developer within the development apparatus is produced in the desired toner density is performed. In addition, the density of a standard toner pattern formed in a non-imaging part is detected and, as a result, the image density during the forming of the standard pattern is ascertained and, based on the detected result thereof, a toner density control target value is corrected. Based on this method, image formation at the desired image density can be performed for a short time period following this correction. Accordingly, forming a standard toner pattern and regularly correcting the toner density control target value in response to the detected result thereof can produce a constant image density.
However, in the image forming apparatuses described in these applications, standard toner patterns must be formed to the extent that the toner density control target value is corrected. Accordingly, and inherent problem thereof is the increased use of the amount of toner not employed in the image formation.